Gypsum boards are known as boards having a gypsum core covered with sheets of paper for gypsum board liner, and are widely used in various kinds of buildings as architectural interior finish materials, because of their advantageous fire-resisting or fire-protecting ability, sound insulation performance, workability, cost performance and so on. In general, the gypsum boards are produced by a continuous slurry pouring and casting process. This process comprises a mixing and stirring step of admixing calcined gypsum, adhesive auxiliary agent, set accelerator, foam (or foaming agent) and so forth with mixing water in a mixing and stirring device; a forming step of pouring calcined gypsum slurry prepared in the device (referred to as “slurry” hereinafter) into an area between upper and lower sheets of paper for gypsum board liner and forming them in a continuous belt-like formation; and a drying and cutting step of roughly severing the solidified continuous belt-like layered formation, drying it forcibly and thereafter, cutting it to be a product size.
Usually, a thin type of circular centrifugal mixer is used as the mixing and stirring device for preparing the slurry. This type of mixer comprises a flattened circular casing and a rotary disc rotatably positioned in the casing. A plurality of material feeding ports for feeding the above materials into the mixer are disposed in a center region of a top cover or an upper plate of the casing, and a slurry outlet port for delivering mixture (slurry) from the mixer is provided in a periphery of the casing or a lower plate (bottom cover) thereof. Typically, a rotary shaft is connected to the disc for rotating the disc, and the shaft is connected with rotary drive means. The upper plate of the casing is equipped with a plurality of upper pins (stationary pins) depending therefrom down to the vicinity of the disc. The disc is equipped with lower pins (movable pins) which are vertically fixed on the disc and which extend up to the vicinity of the upper plate. The upper and lower pins are arranged in radially alternate positions. The ingredients to be mixed are supplied on the disc through the respective feeding ports, and are stirred and mixed while being moved radially outward on the disc under an action of centrifugal force, and then, delivered out of the mixer through the slurry outlet port, which are positioned on the periphery or the lower plate (bottom cover). The mixer with this arrangement is called a pin type of mixer, which is disclosed in, e.g., International Publication of PCT Application No. WO00/56435 (Patent Literature 1).
As regards a method for delivering the slurry prepared in the mixer to the outside of the mixer, the following three kinds of methods are mainly known in the art:
(1) A vertical chute, which is also called as “canister”, is attached to a slurry outlet port provided on an annular wall of the casing, and the slurry on the rotary disc is delivered into the chute under the action of centrifugal force, so that the slurry flowing into the chute is gravitationally spouted onto the sheet of paper (International Publication of PCT Application No. WO2004/026550 (Patent Literature 2));
(2) A tubular passage for transporting the slurry is transversely connected to the slurry outlet port provided on the annular wall of the casing, so that the slurry is spouted onto the sheet of paper with use of a delivery pressure of the mixer (U.S. Patent Publication No. 6,494,609 (Patent Literature 3));
(3) A slurry delivery tubular passage is vertically connected to the slurry outlet port provided on the lower plate of the casing, so that the slurry is gravitationally spouted onto the sheet of paper through the delivery passage (Japanese Patent Laid-Open Publication No. 2001-300933 (Patent Literature 4)).
In general, foam or foaming agent is fed to the slurry in the mixer, in order to regulate or adjust the specific gravity of gypsum board. Proper mixing of the foam or foaming agent in the slurry is considered to be an essential matter in a method for producing lightweight gypsum boards. Therefore, in the method for producing gypsum boards in recent years, a technique for properly mixing an appropriate quantity of foam or foaming agent with the slurry is considered to be especially important. As regards reduction in a supply amount of foam or foaming agent (referred to as “amount of foam” hereinafter) and uniform mixing of the slurry and the foam, it is considered that a relation is very important between a method for feeding the foam or foaming agent to the slurry and a method for delivering the slurry (Patent Literatures 2 and 3).
Each of U.S. Patent Publication No. 6,742,922 (Patent Literature 5) and International Publication of PCT Application No. WO2004/103663 (Patent Literature 6)) discloses a technique intended to attain homogeneous dispersion and distribution of the foam or foaming agent in the slurry with use of a slurry swirling flow in a vertical chute.
A circular fluid passage in the vertical chute is provided with an orifice member in a lower part of the passage, as regards the mixer intended for homogeneous dispersion and distribution of the foam in the slurry by means of a slurry swirling flow generated in the chute. The orifice member has an orifice or constriction (referred to as “orifice” hereinafter). The orifice acts as a fluid resistance against a vertical movement of the slurry, so that the slurry flowing in the chute does not immediately flow down through the chute gravitationally, and therefore, the intratubular swirling flow of slurry is surely created in an intratubular area of the chute.